Translating or extruding in inches
A bit of googling lead me to Metric &amp; Imperial Units in Blender.  

@DemonFate's answer, made me try manually setting the scale to 0.0254, which is 1 inch expressed in metres. I assumed the scale was a conversion from BU to whatever unit I wanted. But from the link above, I realised that when Blender is set to Imperial, the scale is a conversion from feet to whatever unit I want. Hence setting it to 0.08333 does the trick. To get the best precision, type in 1/12 and hit Enter, and Blender is smart enough to calculate it.  

  

There are drawbacks, but I can live with them. The Scale and Location values in the Object tab and in the Properties panel now increments in thou, and all values typed in manually when grabbing or extruding are now taken as inches.